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20CR Climate Indices:SOI

Description:

This mode describes variability of the tropical atmosphere. It is often used as a measure of El Nino/La Nina and is correlated highly with ENSO indices. A high value represents higher surface pressure in the eastern tropical Pacific with more typical easterlies along the equator and is associated with La Nina's. A low value is associated with tropical surface westerlies and El Ninos.

Image:

Correlation of the 20CR-derived SOI timeseries and SLP from the NCEP/NCAR Reanalysis for 1948 to 2011.

Calculation Method:

We used the definition in Ropelewski 1987 MWR. Surface pressure at the grid points nearest to Tahiti and Darwin represent the timeseries at those two locations. Each timeseries is first standardized using the 1981-2011 monthly climatology. The Tahiti minus Darwin timeseries is calculated. This timeseries is then re-normalized using the entire time period for the standardization.